The sintered magnet of Nd—Fe—B family (the so-called neodymium magnet) comprises a combination of Fe, Nd and B which are cheap, abundant and constantly obtainable resources and thus can be manufactured at a low cost and additionally has high magnetic properties (its maximum energy product is about 10 times that of ferritic magnet). Accordingly the sintered magnet of Nd—Fe—B family has been used in various kinds of articles such as electronic instruments and in recently adopted in motors and electric generators for hybrid cars.
On the other hand, since the Curie temperature of the sintered magnet of Nd—Fe—B family is low (about 300° C.), there is a problem the sintered magnet of Nd—Fe—B family would be demagnetized by heat when heated to a temperature exceeding a predetermined temperature under a certain circumstantial condition in its adopted articles. In addition there is further problem that the magnetic properties would be extremely deteriorated by defects (e.g. cracks etc.) or strains in grains of the sintered magnet which are sometimes caused when the sintered magnet is machined to a desired configuration suitable for a particular article.
For solving these problems mentioned above, it is known to improve or recover the magnetizing properties and coercive force by arranging rare earth elements selected from Yb, Eu and Sm in a processing chamber under a condition mingled with a sintered magnet of Nd—Fe—B family, evaporating rare earth elements by heating the processing chamber, attaching the evaporated atoms of the rare earth elements into the sintered magnet, and further diffusing the attached atoms into the grain boundary phases of the sintered magnet in order to homogeneously introduce desired amount of the rare earth elements into a surface of the sintered magnet and the grain boundary phases (Patent Document 1 mentioned below).
It is also known that Dy and Tb of the rare earth elements have the magnetic anisotropy of 4f electron larger than that of Nd and a negative Stevens factor similarly to Nd and thus can remarkably improve the grain magnetic anisotropy of principal phase. However since Dy and Tb take a ferrimagnetism structure having a spin orientation negative to that of Nd in the crystal lattice of the principal phase, the strength of magnetic field, accordingly the maximum energy product exhibiting the magnetic properties is extremely reduced. Thus it has been proposed to homogeneously introduce a desired amount of Dy and Tb especially into the grain boundary phases in accordance with the method mentioned above.
[Patent Document 1] Japanese Laid-open Patent Publication No. 296973/2004 (e.g. refer to descriptions in claims thereof)